Crude petroleum oil and gaseous hydrocarbons are produced by extraction from subterranean reservoirs. In reservoirs with enough natural pressure, oil and gas flows to the surface without secondary lift techniques. Often, however, other methods are required to bring them to the surface. These include a variety of pumping, injection, and lifting techniques used at various locations, such as at the surface wellhead (e.g., rocking beam suction pumping), at the bottom of the well (e.g., submersed pumping), with gas injection into the well casing creating lift, and other techniques. Each technique results in oil and gas emerging from the well head as a multiphase fluid with varying proportions of oil, water, and gas. For example, a gas lift well has large volumes of gas associated with the well. The gas-to-oil volume ratios can be 200 cubic feet or more of gas per barrel. Large measurement uncertainties may occur, depending upon the methods used.
The measurement of water in petrochemical products is a common practice in the petroleum industry. This measurement is frequently done in combination with oil well testing to assist in optimizing oil production from a single oil well or a series of oil wells. The measurement may also be performed during the transfer of crude petroleum oil, as occurs during the production, transport, refining, and sale of oil. Specifically, it is well known to a person having ordinary skill in the art of petroleum engineering that crude petroleum oil emerging from production wells can contain large amounts of water, ranging from generally about 1% to as high as about 95% water. This value is known as the water cut (“WC”). Multiphase measurements typically provide an oil company and other stakeholders with the amount of gas, oil, and water and the average temperature, pressure, gas/oil ratio, and gas volume fraction that a well produces in a day.
Typical techniques to determine the water percentage or water cut is to use a capacitive, radio frequency, or microwave analyzer to perform the in-line monitoring of the oil and water mixture within a pipeline. U.S. Pat. No. 4,862,060 to Scott, entitled “Microwave Apparatus for Measuring Fluid Mixtures”, discloses microwave apparatuses and methods which are most suitable for monitoring water percentages when the water is dispersed in a continuous oil phase. U.S. Pat. No. 4,862,060 is hereby incorporated by reference as if fully set forth herein.
It is well known to electrical engineers and particularly microwave engineers that the frequency of a radio frequency (RF) oscillator can be “pulled” if the oscillator sees an impedance which is different from the ideal matched impedance. That is, the RF oscillator is pulled or shifted from the frequency of oscillation which would be seen if the oscillator were coupled to an ideal impedance-matched pure resistance. Thus, varying the load impedance may cause the oscillator frequency to shift. U.S. Pat. Nos. 4,862,060 and 4,996,490 describe the application of load pull oscillators in detail.
For example, an unbuffered RF oscillator is loaded by an electromagnetic propagation structure which is electromagnetically coupled, by proximity, to a material for which real-time monitoring is desired. The net complex impedance seen by the oscillator varies as the characteristics of the material in the electromagnetic propagation structure varies. As this complex impedance changes, the oscillator frequency varies. Thus, the frequency variation, which can easily be measured, reflects changes in density (e.g., due to bonding changes, additional molecular chains, etc.), ionic content, dielectric constant, or microwave loss characteristics of the medium under study (i.e., the multiphase fluid). These changes “pull” the resonant frequency of the oscillator system. Changes in the magnetic permeability of the medium will also tend to cause a frequency change, since the propagation of the RF energy is an electromagnetic process which is coupled to both electric fields and magnetic fields within the transmission line.
Load-pulled oscillators, which make use of this effect, are an important technique for RF monitoring. A free-running oscillator, typically at VHF or higher frequencies, is electromagnetically coupled to some environment which is to be characterized or analyzed. For example, an unknown oil/water/gas composition may flow through a coaxial probe element. Since the oscillator is not isolated from the environment being measured, changes in that environment pull the frequency of oscillation. By monitoring shifts in the frequency of oscillation, changes in the environment being monitored may be measured with great precision. For example, in compositional monitoring of wellhead flows of oil/gas/water mixtures, the environment being monitored is a medium having a variable composition and changes in the composition are seen as shifts in the oscillation frequency for a given tuning voltage.
The oil industry covers every possible climate condition across the world. This requires temperature compensation circuitry to be included in any electronic multiphase analyzer. By way of example, U.S. Pat. Nos. 4,862,060, 4,996,490, 5,748,002, 6,593,753, and others describe the use of load pulled oscillators that use a plate heated to 80 degrees Celsius under the temperature sensitive oscillator portion of the circuitry to eliminate the ambient temperature from being seen by the components. This method is effective but forces the use of 30 watts of power in a cold environment. This prevents the use of intrinsically safe circuits and is demanding on the power supply. U.S. Pat. Nos. 4,862,060, 4,996,490, 5,748,002, 6,593,753 are hereby incorporated by reference as if fully set forth herein.
U.S. Pat. No. 6,867,599 describes a circuit and method to compensate a load pulled oscillator for temperature variation by using a reference load and tuning the oscillator with a voltage to achieve the same frequency for a given reference load. U.S. Pat. Nos. 6,687,599 4,862,060 is hereby incorporated by reference as if fully set forth herein. Although effective, this approach has several disadvantages. Any noise on the lines for the tuning circuits from the microprocessor or other noise sources will alter the ability to set the operating point. Changing the external capacitance in the oscillator feedback loop using a varactor might alter the load pull characteristic impedance. The added circuitry required to accomplish the load switching increases the attenuation between the oscillator and the load, thereby reducing the ability of the oscillator to change frequency. This reduces the advantage that the load pull characteristic provides.
Therefore, there is a need for improved systems and methods for measuring the water cut of a multiphase fluid. In particular, there is a need for a multiphase fluid analyzer capable of taking accurate water cut measurements across a wide spectrum of operating temperatures. More particularly, there is a need for apparatuses and methods to effectively correct the ambient temperature effects for a load pulled oscillator which reduces the complexity of the method described in U.S. Pat. No. 6,867,599.